The KRAS oncogene is mutated in ~95% of pancreatic ductal adenocarcinomas (PDAC). Mutated Kras oncoproteins are locked in a constitutively active, GTP-bound state that activates multiple effector signaling cascades leading to PDAC initiation, maintenance, and progression. Strategies of directly targeting KRas have not been successful over the last three decades, so inhibition of KRas effector signaling pathways appears to be the most promising direction at the moment for advancement to the clinic. In particular, considerable efforts and interests are now focused on inhibitors of the Raf-MEK-ERK mitogen-activated protein kinase (MAPK) cascade. However, Raf and MEK inhibitors have shown limited activity in RAS-mutant cancers, largely owing to the rapid emergence of the escape mechanism of ERK reactivation. These findings have prompted the development of ERK inhibitors. Among them, BVD-523, a small molecule that targets ERK1 and ERK2 in the sub-nanomolar range, is the leading compound entering oncology clinical trials. PDXs are the most clinicallyrelevant model for testing the efficacy of BVD-523 and identifying potential intrinsic (i.e cell-autonomous) and extrinsic (i.e tumor microenvironmental) resistance mechanisms that can be exploited for development of more potent combinatorial regimens. Our Research Project 2 aims at testing the efficacy of BVD-523 using our extensive repository of 105, clinically-annotated PDAC patient-derived xenografts (PDX). In our proposal, we plan to develop effective ERK inhibitor-based therapies in conjunction with predictive biomarkers by leveraging our extensive repertoire of PDXs, which can be rapidly advanced into clinical trials. To achieve this objective, we propose the following three aims: 1) undertake proteogenomic and functional characterization of PDXs of pancreatic cancer, 2) test combination approaches that overcome tumor-intrinsic ERK inhibition resistance mechanisms, and 3) develop therapeutic combination approaches that overcome tumor-extrinsic ERK inhibition resistance mechanisms. Our goal is to identify combinations that can be quickly advanced into clinical trials in pancreatic cancer and predictive biomarkers that can be used to enrich patients population for the study.